Revert "Fix an off-by-one on registration timeout handling."

This seemingly breaks (occasionally timing users out a second early), possibly
due to rounding issues on time (User::age and Time() both return int, but a
whole second is comprised of many ms).

The root cause needs investigation, but we cannot leave a potentially breaking
bug in the tree.

This reverts commit 4414d644a163f3906f90b35186e07ce0383161f4.
Merge pull request #930 from rburchell/revertfix

Revert "Fix an off-by-one on registration timeout handling."